Optical brighteners compensate for the yellow cast produced by light or heat degradation of the polymer over time. Typical optical brighteners fluoresce upon irradiation with ultraviolet light emitting visible light, typically bluish in hue, replacing the light that would have been lost and thereby enhancing brightness. Optical brighteners mask undesirable tints, such as yellow tint from recycled products, and/or modify the fluorescent nature of certain color imparting pigments.
4,4'-Bis(benzoxazol-2-yl)stilbene has been used extensively as an optical brightener for synthetic polymeric materials such as polyesters, e.g., poly(ethylene terephthalate), and polyamides, e.g., nylon 6,6. 4,4'-Bis(benzoxazol-2-yl)stilbene, also having the chemical name 2,2'-(1,2-ethenediyl di 4,1-phenylene)bis benzoxazole, typically is incorporated into such synthetic polymers by the technique known as "mass brightening" wherein the optical brightener (or whitener) is added to a melt of the polymer prior to its conversion into a shaped article such as a fiber or filament suitable for use in the textile industry. Mixtures of unsubstituted and substituted 4,4'-bis(benzoxazol-2-yl)stilbene compounds are more effective than is the unsubstituted compound for certain uses, e.g., for topical application to textile materials such as fibers, filaments and fabrics prepared from polyesters and in the manufacture of photographic paper wherein the lower melting point of the mixture is advantageous.
A particularly efficient method for the preparation of unsubstituted 4,4'-bis(benzoxazol-2-yl)stilbene comprises the steps of (1) reacting an alkyl p-toluate with o-aminophenol to produce 2-(p-tolyl)benzoxazole and (2) heating 2-(p-tolyl)benzoxazole with sulfur to dehydrodimeize the 2-(p-tolyl)benzoxazole and produce 4,4'-bis(benzoxazol-2-yl)stilbene. Although it is eminently suited for the synthesis of unsubstituted 4,4'-bis(benzoxazol-2-yl)stilbene, the toluate ester method is not especially useful for preparing substituted 4,4'-bis(benzoxazol-2-yl)stilbene, e.g., compounds wherein one or both of the benzoxazole rings are substituted with an alkyl group, or mixture of such compounds with unsubstituted 4,4'-bis(benzoxazol-2-yl)stilbene. Such substituted compounds and mixtures may be prepared by first synthesizing 4,4'-stilbenedicarboxylic acid or an ester or diester thereof and reacting the 4,4'-stilbenedicarboxylic acid or ester with a mixture comprising unsubstituted o-aminophenol and at least one substituted o-aminophenol such as 2-amino-4-methylphenol. The product consists of a mixture unsubstituted 4,4'-bis(benzoxazol-2-yl)stilbene, mono-substituted 4,4'-bis(benzoxazol-2-yl)stilbene, i.e., 4-(5-methylbenzoxazol-2-yl)-4'-(benzoxazol-2-yl)stilbene, and disubstituted 4,4'-bis(benzoxazol-2-yl)stilbene, i.e., 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene.
The processes described in U.S. Pat. No. 4,921,964 are typical of the known processes for the preparation of mixtures of 4,4'-bis(benzoxazol-2-yl)stilbene compounds. That patent describes the preparation of unsubstituted 4,4'-bis(benzoxazol-2-yl)stilbene as well as the aforesaid mixture of 4,4'-bis(benzoxazol-2-yl)stilbene, 4-(5-methylbenzoxazol-2-yl)-4'-(benzoxazol-2-yl)stilbene, and 4,4'-bis(5-methylbenzoxazol-2-yl)stilbene, by reacting dimethyl 4,4'-stilbenedicarboxylate with o-aminophenol or a mixture of o-aminophenol and 2-amino-4-methylphenol. The reaction typically is carried out at a temperature of about 200 to 250.degree. C. in the presence of tin or titanium catalyst.